The gonadotropins follitropin (follicle-stimulating hormone, FSH) and chorionic gonadotropin, (CG), lutropin (luteinizing hormone, LH), and thyrotropin (thyroid-stimulating hormone, TSH) comprise the family of glycoprotein hormones. Each hormone is a heterodimer of two non-covalently linked subunits: alpha and beta. Within the same species, the amino acid sequence of the alpha-subunit is identical in all the hormones, whereas the sequence of the beta-subunit is hormone specific (Pierce, Ann. Rev. Biochem. 50:465-495 (1981)). The fact that the sequences of the subunits are highly conserved from fish to mammals implies that these hormones have evolved from a common ancestral protein (Fontaine, Gen. Comp. Endocrinol. 32:341-347 (1977)). The ubiquitous presence of the glycoprotein hormones across the spectrum of animals also indicates significant potential for the hormones or any modified versions of the hormones to be used in various species.
Previous studies with modified glycoprotein hormones have revealed encouraging data. For example, in addition to providing modified glycoprotein hormones with increased activity, further mutations have demonstrated an increase in receptor affinity binding (see e.g. WO 2005/089445 and WO 2005/101000). However, while affinity was increased, studies demonstrated that modified glycoprotein hormones were cleared as quickly if not faster than their wild type counterparts. In order to generate a clinically useful superagonist with enhanced activity, modified glycoprotein superagonist should have an improved biological half-life in addition to improved receptor binding affinity. However, previous attempts to further modify glycoprotein hormones to increase half-life and improve bioavailability have been less than satisfactory and instead the modified glycoprotein hormones demonstrated only an attenuated response.